1) Field of the Invention
The present invention relates to the fabrication of integrated circuit devices on semiconductor substrates, and particularly to a method for forming gate structures for salicide processes, and more particularly relates to a method for making sub-quarter-micrometer gate electrodes having a "mushroom shape".
2) Description of the Prior Art
Advances in the semiconductor process technologies in recent years have dramatically decreased the device feature size and increased the circuit density and performance on integrated circuit chips. The field effect transistor (FET) is used extensively for Ultra Large Scale Integration (ULSI). These FETs are formed using polysilicon gate electrodes and self-aligned source/drain contact areas.
The conventional FETs are typically fabricated by patterning polysilicon gate electrodes over a thin gate oxide on a single crystal semiconductor substrate. The gate electrode structure is used as a diffusion or implant barrier mask to form self-aligned source/drain areas in the substrate adjacent to the sides of the gate electrode. The distance from the source junction to drain junction under the gate electrode is defined as the channel length of the FET.
Advances in semiconductor technologies, such as high resolution photolithographic techniques and anisotropic plasma etching, to name a few, have reduced the minimum feature sizes on devices to less than a quarter-micrometer. For example, FETs having gate electrodes with widths less than 0.25 micrometers (um), and channel lengths that are less than the gate electrode width are currently used in the industry.
However, as this downscaling continues and the channel length is further reduced, the FET device experiences a number of undesirable electrical characteristics. One problem associated with these narrow gate electrodes is the high electrical sheet resistance which impairs the performance of the integrated circuit. One method of circumventing this problem is to form on the gate electrode a metal silicide layer that substantially reduces the sheet resistance of the polysilicon gate electrode, and also the local electrical interconnecting lines made from the same polysilicon layer. A typical approach is to use a salicide process. In this process the polysilicon gate electrodes are patterned over the device areas on the substrate. Insulating sidewall spacers are formed on the sidewalls of the gate electrodes, and source/drain areas are implanted adjacent to the gate electrodes. Using the salicide process, a metal is deposited over the polysilicon gate electrodes and the self-aligned source/drain areas, and sintered to form a silicide layer on the polysilicon gates and silicide contacts in the source/drain areas. The unreacted metal on the insulating layer is selectively removed. Unfortunately, the formation of these salicide gate electrodes can result in undesirable effects, such as residual metal or silicide stringers extending over the narrow spacers causing electrical shorts between the gate electrodes and the source/drain areas.
A second problem, the inventors found, results from forming titanium silicide on sub-quarter-micrometer FETs. The problem is that it is difficult to form low sheet resistance silicide on these sub-0.25-micrometer (.mu.m) gate lengths. One method to circumvent this problem is to use a cobalt or a nickel silicide to replace the titanium silicide. U.S. Pat. No. 5,731,239 (Wong) (Assigned to same assignee) describes an alternative method to improve the sheet resistance is to amorphize the polysilicon layer by ion implantation prior to forming the titanium silicide.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 5,650,342 (Satoh) shows a T shaped gate. U.S. Pat. No. 5,434,093 (Chau) shows an inverted spacer transistor. U.S. Pat. No. 5,565,383 (Sakai) shows a gate and salicide process. U.S. Pat. No. 5,731,239 (Wong) shows a self-aligned silicide narrow gate process. U.S. Pat. No. 5,726,081 (Lin) shows a T shaped gate electrode. U.S. Pat. No. 5,710,450 (Chau) shows another salicide process. U.S. Pat. No. 5,688,704 (Liu) teaches a T shaped gate formed without a spacer process.
However, the sub 0.25 .mu.m Salicide process can be further improved. Therefore, there is still a strong need in the semiconductor industry for making sub-quarter-micrometer gate electrodes having lower sheet resistance using improved silicide techniques, and for controlling manufacturing costs by reducing the number of photoresist masking steps and other processing steps.